CRTs emit visible light by striking phosphors on the faceplate with electrons and transferring the electron's kinetic energy to light, heat, and other forms of energy, e.g., X-radiation. The frequency spectrum of photon emission is a property of the phosphor material. Besides the type of target material, the probability of emission of a photon of X-ray frequency depends on the accelerating potential (anode voltage) and the number of electrons striking the phosphor in a unit of time (“anode current” or “beam current”).
In high performance television receivers and computer displays, the perceived light output is a function of the amount of beam current and high voltage power. In such receivers and monitors, it is desirable to set the operating point as high as possible, while strictly complying with X-ray protection regulations. Frequently, when component tolerances are considered, the high voltage operating point must be lower than desired to ensure the protection circuit will always trip at acceptable levels, but will not activate in normal operation, or during transients, such as channel change.
Each model of CRT has an individual maximum acceleration voltage and beam current that corresponds to a maximum allowable X-radiation emission. In many television receivers and computer displays, the anode current for “normal operation” is adjusted to always be below a maximum, worst case current, ensuring the legal regulation regarding the maximum allowable X-radiation emission is not violated. Since, at any possible operating point, and under any possible fault condition, no television or computer display instrument is allowed to emit X-radiation in excess of published limits, many television receivers and computer displays using CRTs contain “X-ray protection circuits”, or XRPs. In FIG. 1, the curve (1) for maximum allowable acceleration voltage for a television receiver model is illustrated as a function of the beam current. The television receiver has a high voltage circuit tolerance of 2.5 kV and an X-ray protection circuit tolerance of 2.3 kV is illustrated. Curves 2, 3 and 4 show the nominal, upper and lower trip voltages of the XRP circuit, respectively, and curves 5, 6 and 7 show the nominal, upper and lower voltages of the high voltage transformer, respectively, resulting from the component tolerances. To establish compliance with legal requirements, the nominal operating high voltage is set approximately 9.5 kV below the maximum value. The tolerance in the high voltage circuit may be reduced by selection of precision components. High voltage tolerance may be nearly eliminated by measuring the high voltage and providing a feedback loop to regulate the high voltage. These methods, although expensive and not well suited for mass production, have been used in television receiver and video monitor design.
One type of XRP causes the television picture to become “not viewable” when a fault is detected. One method to achieve this is to force the horizontal scan frequency high, resulting in a picture that “rolls”. The disadvantage of this method is that the television receiver continues to operate with the fault present.
Another, more recent type of XRP interrupts the horizontal deflection. In televisions and monitors where the High Voltage transformer is an integral part of the deflection yoke scan current, turning off the horizontal deflection may result in ceasing generation of the high voltage, which is the accelerating potential for the electron beam, as mentioned above. In recent television receivers, the video and color processing, audio, sync separator, and timing signals for the horizontal and vertical deflection, are commonly contained in an Integrated Circuit, or chipset. Such so-called “One-Chips” often have an “X-Ray Protection” pin, which, when activated, removes the drive to the horizontal deflection. While stopping the horizontal deflection has the advantage of removing the source of the electrical field that allows acceleration of the electron beam, it may be difficult to implement, in practice. Because of the interest in larger receivers and monitors, and more light output, the horizontal scan circuitry is required to operate under a wide range of load conditions. One way to drive the horizontal scan more efficiently is to provide feedback regarding the load to the horizontal drive section. This “proportional drive” concept increases the base drive to the horizontal output switch when the load current is increased. The horizontal scan circuit, however, is a resonant circuit, by nature, and may continue to try to oscillate, even at very low drive levels. If no distinct “X-Ray protection” that stops the horizontal deflection is incorporated, it may, in the event of a fault, be difficult to remove the horizontal drive completely.
In order to activate in the event of excess X-radiation, many XRP circuits monitor an “image” of the anode voltage. Typically, an additional winding on the high voltage transformer generates this image, which follows, by virtue of magnetic coupling, the high voltage at the anode of the CRT at a fixed ratio determined by the number of windings. The additional or “X-Ray Protection” winding voltage may be rectified and compared to a reference to determine whether or not a fault has occurred. If the “XRP voltage” is below a reference level, the television instrument is assumed to be in normal operation. If the “image-voltage” representing the anode voltage exceeds a set reference level, which indicates exceeding of the admissible high voltage, or the rate of X-rays emitted by the CRT, the X-ray protection circuit must cause the instrument to cease “normal operation”.
One method to compensate for XRP circuit tolerances mentioned above is to add a variable resistor, or potentiometer, to adjust either the reference voltage to the XRP circuit, or the “image voltage” generated in the additional winding. During production alignment, the scan supply may be increased until the target high voltage for XRP circuit activation is reached. The potentiometer is then adjusted until the XRP output changes its state, indicating a fault has occurred. However, using potentiometers may cause many problems. As a mechanical device, it is subject to contamination, vibration, thermal expansion and contraction, and aging. Often, to reduce the chance the alignment will be changed adhesives or protective cases are used.
Another method to reduce the effects of circuit tolerances is disclosed in U.S. Pat. No. 6,075,687, issued Jun. 13, 2000, to Cheng et al. The method presented in Cheng uses sampling and converting to digital of a rectified “image voltage”, generated in an additional winding. A microprocessor periodically polls the digital value of the voltage corresponding to the high voltage from the analog to digitized converter and compares the result with a high and a low limit, stored in memory, which have been calibrated during production alignment. On crossing any of the upper or lower limits, the microprocessor issues a signal, cutting off the power supply to the high voltage transformer.
An “open loop” method of reducing the high voltage tolerance during production alignment is to adjust the feedback of the power supply, which energizes the primary winding of the high voltage, until the desired target value of high voltage is obtained. This alignment method, however, may increase the tolerance in any other secondary supplies for other circuits.